As is well known, mechanical transmissions quite typically include housings containing rotating shafts, gears and other rotating components. Provision is made for lubricating bearings, the interfaces of meshing gears, or the like with lubricating oil. Frequently, the rotational speed of the parts receiving the oil is such that the oil is broken down into fine droplets to generate a mist within the transmission.
Such oil must be retained within the transmission if lubrication costs are to be minimized and damage to the transmission for lack of lubricant avoided. Further, discharge into the surrounding environment will generally be undesirable.
At the same time, it is desirable that transmissions be vented so that pressures do not build up within the same as the transmission heats up from ambient after it has been put in operation as such pressure could drive lubricant out of the housing through seals or the like, resulting in a loss of lubricant and ultimate failure of the transmission due to seizure of parts. In the normal case, such venting does not pose a particular problem since basically all that is involved is disposing a vent at a location isolated, usually by baffles, from the area in which lubricant may be entrained in the atmosphere within the transmission. In essence, the transmission merely "breathes" through such a vent with very little gas exiting or entering the vent.
There are, however, instances where the transmission will be operating under a positive internal pressure in relation to the ambient. One such example is transmissions utilized in power units such as auxiliary power units or emergency power units employed in aircraft. Not infrequently a gas turbine will be utilized to drive hydraulic pumps and/or electrical generators via a transmission. While the power takeoff from the turbine to the transmission can be taken from the turbine wheel side of the gas turbine engine, because that environment is relatively more hostile due to hot exhaust gases and the like, it is more common that the power takeoff be taken from the compressor side of the gas turbine engine. As a consequence of this in many gas turbine designs, there exists a leakage path from the output side of the compressor through the engine rotor and ultimately to the transmission to which it is connected. As a consequence, compressed air from the compressor of the engine will be continually leaking into the transmission during operation and such gas must be vented from the transmission continually without discharging lubricant so as to prevent the buildup of pressure within the transmission.
The present invention is directed to solving the above problem.